My outlets are the old kind where the wires are pushed into the back for contact. How do I change these?

Some have 3 groups of wires pushed into the back. How do I change these out? I've noticed that the new oulets only have room for 2 wires (because these wires are a little thicker than the hole that can be used on the back of the outlet for connection). I know that I will need to install boxes because these did not have any. Any advice besides hire an electrician? I live in the country so we are not required by law to contract out we can do it ourselves.